Liam Smith twice blasted Chris Eubank down to the canvas and finished their fight in four astonishing rounds at the Manchester Arena.
After a dominant third round from Eubank, Smith came flying out of the traps in the fourth, flooring his opponent for the first time 45 seconds into the round.
Eubank was badly hurt and Smith put him down a second time within seconds, with the referee leaping in to call a halt even as Eubank tried to fight on.
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Smith's long rivalry with Eubank Jr dates back more than seven years to a bitterly contested series of sparring sessions.
It left the Liverpool man convinced he was the better fighter, even once he'd stepped up to middleweight. Those suggestions were imperiously dismissed by Eubank Jr, only enraging the former WBO titlist further.
Despite the fractious build-up to this contest, Smith was the crowd favourite as he entered a raucous Manchester Arena, packed out with 18,000 in attendance.
It was a setting he'd craved, a stage that gave him the chance to prove whether he could recapture his previous status as a world champion once again.
Yet unfazed by the hostile atmosphere, Eubank left Smith waiting for a long time in the ring as he processed in, stepped up to the ring apron and looked out to the arena, before easily vaulting over the top rope in the manner of his famous father.
Smith, representing a boxing dynasty of his own, is one of four brothers who have all won British titles, all challenged for world titles and two of whom have previously been world champions.
His pride and potentially his boxing future were on the line in this fight. But he answered all the questions Eubank had asked of him with that extraordinary ending.
Smith wasn't supposed to be the power puncher in this particular fight. He was though a ruthless finisher.
Story of the fight
To begin Eubank doubled his jab, then threw that left to the body. Smith edged forward with his gloves high. He launched his right to the body. But stepping in again a punch to the head caught him flush.
Smith though didn't give away too many openings. Eubank was active in that first round, letting his hands go but generally made few mistakes, only receiving glancing blows as his right dug to the body again.
Smith's corner called for "educated pressure" and he provided it in the second round. Coming on, he still slipped a three-punch combination and slugged a right to Eubank's head. Smith made Eubank throw but didn't let him land too much.
Eubank therefore moved off, looking to box, but conceding ground.
Smith ducked a wild left hook and resumed his front foot approach. Eubank snapped his jab out, keeping Smith off and clipped him with a right, before then bringing in dangerous uppercuts with his back hand. Those uppercuts were a crucial new addition to his work in the third round. They swung that round his way and all of a sudden he seemed to take a strong hold of the fight.
But Smith changed it all in the fourth round. He backed Eubank into a corner and unloaded a combination. Connecting hard, he stunned Eubank and dropped him heavily to the canvas.
It was an expertly executed combination, a left uppercut followed by a savage left hook.
Eubank rose bravely but was unsteady and a right put him down again immediately.
He rose again, trying to barrel through the arms of the referee on instinct now but Victor Loughlin would not let him take further damage, ending it at 1:09 of the fourth round.
What they said
Liam Smith said: "I don't know if it was one shot or an accumulation, but I told you all week.
"A lot got made of Chris' great chin, I've got a great chin, but there's many fighters with better chins who have been knocked out in the past.
"I told you all week, don't be surprised, nobody can not be hurt."
Chris Eubank Jr said: "Big congratulations to him. He caught me with a great shot.
"The build-up was the build-up, it got a bit ugly at the end. I regretted that. I respect you, I respect your family, I always have.
"If the fans want to see a rematch, we can get it on at Anfield.
"Big respect, man."
Repeats of Chris Eubank Jr vs Liam Smith are on Sunday January 22 at 8am and 3pm. Book it now if you are a Sky TV subscriber or a Non-Sky TV subscriber.